I picked up a 7155 receiver and when powering up, the relay clicks in after a few seconds and there is a distinct low level hum from both speakers, with or without inputs connected. A little louder in the right channel. I think it is 120hz.

After about 10 minutes the hum diminishes to a point where I cannot hear it when 8 feet away from the speakers. I would like to fix it though.

I have a donor 7155 minus itís volume/balance control.
I have a soldering iron and a DVM and access to the circuit diagram.

Any ideas where I should start looking, swapping etc to try and fix it?

Iíve gained access to the amplifier board and find no swelling or leaking on any of the mentioned caps, but there is some scorching of the board in the vicinity of C907(photo attached). The center of the scorched area is at Q903.
I have a donor 7155 amp and the amp board looks very clean and no sign of overheating. The mentioned caps have been removed from the donor board ready to swap into the faulty one.
Should I address any further components in the overheated area?
Many thanks
Peter

Just purchased a dozen caps from Mouser and included replacements for all the 9xx caps in the scorched area.
Apparently someone has soldered a diode across C917. Photo attached. I have no idea why, maybe it is a band aid for a problem? Is it related to a problematic cap that I am going to replace or something else?
The donor board that I have is very clean and all components intact.
Any help would be much appreciated.
Thanks
Peter

Just purchased a dozen caps from Mouser and included replacements for all the 9xx caps in the scorched area.
Apparently someone has soldered a diode across C917. Photo attached. I have no idea why, maybe it is a band aid for a problem? Is it related to a problematic cap that I am going to replace or something else?
The donor board that I have is very clean and all components intact.
Any help would be much appreciated.
Thanks
Peter

Thanks for your help.
The identical donor amp does not have the diode, and my knowledge of why it would be there is nonexistent. Or how it affects the circuit.
I’ve been scrutinizing for dry joints.
The hum I experience is quite minor when the amp has been on for a few minutes. I’d rather not have it, and if this doesn’t fix it, I will tolerate it. I bought it along with some Thiel speakers that I have fallen in love with, and when I first heard the system, didn’t even notice the noise. Otherwise, I like it.
Peter